1. Field of the Invention
The present invention relates to a high-frequency filter for use in a superhigh frequency band (generally from 1 to 100 GHz) such as of microwaves and millimeter waves, and more particularly to a high-frequency filter having a microwave integrated circuit structure and capable of electronically controlling filter characteristics such as transmission characteristics, in particular, band characteristics.
2. Description of the Related Art
High-frequency filters are widely used as a functional device indispensable for introducing and extracting desired signals and suppressing and removing unwanted signals in transmission/reception apparatus in various radio communication facilities, optical fiber high-speed transmission apparatus, and measuring devices in association therewith.
Heretofore, high-frequency filters for use in the microwave band and higher frequency bands are generally constructed using metal waveguides or dielectric resonators. In recent years, high-frequency filters having a microwave integrated circuit structure are also finding growing use for their small size. However, high-frequency filters of a microwave integrated circuit structure generally have fixed filter characteristics and suffer limitations in general-purpose applications. There have been proposed high-frequency filters of a microwave integrated circuit structure capable of electronically controlling filter characteristics, as reported in academic societies.
FIG. 1 shows a conventional high-frequency filter having a microwave integrated circuit structure. As shown in FIG. 1, the high-frequency filter basically has a resonator comprising a transmission line formed on substrate 1 which is made of, for example, a dielectric material. In FIG. 1, the transmission line comprises microstrip lines. Specifically, the transmission line includes a plurality of (e.g., three) signal lines 2 and input and output lines 3, 4, each made of a metal conductor, arranged at transversely spaced intervals on one main surface of substrate 1. Signal lines 2 are sandwiched between input and output lines 3, 4, and signal lines 2 and input and output lines 3, 4 are closely positioned so that they are electromagnetically coupled. A ground conductor, i.e., a metal conductor for grounding purpose, is placed as a ground plane on the other main surface of substrate 1.
Each of signal lines 2 is divided into signal line segments 2a, 2b that are connected to each other by a voltage-variable capacitance element such as variable-capacitance diode 6, for example. A control voltage is applied to variable-capacitance diodes 6 via LPF (low-pass filter) 5. The ends of signal line segments 2a remote from respective variable-capacitance diodes 6 are connected to the ground conductor on the other main surface of substrate 1 through respective via holes (through electrode holes) 7 or the like. LPF 5 serves to block high-frequency signals and pass the control voltage therethrough.
With the high-frequency filter, if the resonant frequency has a wavelength of xcex, then the length of each of signal lines 2, which comprises a microstrip line, is set to approximately xcex/4, making each of signal lines 2 function as a resonator. Since the variable-capacitance diode 6 is inserted in each microstrip line, i.e., signal line 2, and the capacitance across the variable-capacitance diode 6 varies depending on the control voltage applied thereto, the resonant frequency of the resonator is variable. This resonator structure can be constructed in a smaller size than dielectric resonators, allowing each resonator to be used in general-purpose applications and to be practical in use.
Because the microstrip lines, i.e., signal lines 2, are arranged at transversely spaced intervals, thus connecting the resonators in cascade, the attenuation slope in the band characteristics of the high-frequency filter can be made steep by equalizing the resonance frequencies of the respective resonators. The high-frequency filter can therefore be used as a practical high-frequency filter. If input and output lines are connected to each individual resonator, i.e., each signal line 2, then the resultant high-frequency filter has a relatively gradual attenuation slope.
With the conventional high-frequency filter described above, the end of each signal line 2 as a microstrip line remote from variable-capacitance diode 6 is connected to the ground conductor on the other main surface of substrate 1 through via hole 7 which needs to be formed by a perforating process. In addition, LPF 5 is required to isolate the high-frequency signal and the control voltage from each other. For these reasons, the conventional high-frequency filter suffers drawbacks that make it difficult to produce the high-frequency filter in smaller sizes with increased accuracy at increased productivity. Specifically, the inductive component tends to increase due to the conductor length (line length) through each via hole 7, thereby degrading the high-frequency characteristics of the filter, and the characteristics of the filter are liable to differ owing to manufacturing errors of via holes 7.
It is therefore an object of the present invention to provide a high-frequency filter which has a steep attenuation slope, has filter characteristics electronically controllable, can be manufactured with increased accuracy at increased productivity, and is suitable for small-size designs.
According to the present invention, the above object can be achieved by a high-frequency filter comprising a substrate, a metal conductor disposed on a first main surface of the substrate, a resonator comprising a transmission line of a coplanar structure which is made of the metal conductor, and input and output lines disposed on a second main surface of the substrate transversely across the resonator and electromagnetically coupled to the resonator.
The substrate comprises a dielectric substrate, for example. The resonator as the transmission line of the coplanar structure is disposed on a first main surface of the substrate, and input and output signal lines extending across the resonator and electromagnetically coupled to the resonator are disposed on a second main surface of the substrate. The high-frequency filter produces a new resonant (frequency) point determined by the opposite ends of the resonator (i.e., transmission line) and points where the input and output lines cross the resonator. Since the length determining the resonant point is shorter than the transmission line, the frequency due to the resonant point is higher than the resonant frequency due to the transmission line (i.e., resonator). Therefore, an attenuating pole is produced in a high-frequency range of band characteristics of the resonator, with the result that a steep attenuation gradient is developed in the band characteristics of the high-frequency filter.
If variable-reactance elements such as variable-capacitance diodes are connected to the resonator, then the resonant frequency can be changed, so that the filter characteristics can electronically be controlled.
Since it is not necessary to provide via holes or the like, the high-frequency filter according to the present invention can be fabricated with increased accuracy at increased productivity, and can be reduced in size.